1. Field of the Invention
The present invention relates to inhibiting over-current in a power amplifier driving a load in a BTL configuration.
2. Description of the Related Art
In an audio power amplifier for STL operation, a load, such as a speaker, is connected to bridge between two output terminals (+OUT, −OUT). Each output amplifier respectively connected to each output terminal outputs a signal having an opposite phase to each other with respect to the load to drive the load. In this power amplifier, if the two output terminals +OUT and −OUT are inadvertently connected (shorted) to a high voltage supply Vcc or a low voltage supply (ground or GND), a particularly large current flows to a power transistor in the output amplifier section causing failure. Thus, heretofore, detecting currents higher than a predetermined amount flowing to the power amplifier and controlling the operation of the amplifier were performed.
FIG. 1 shows a configuration of over-current prevention of the related art in a BTL-type audio power amplifier. One of two output amplifier sections in the power amplifier driving a load L in a BTL configuration has a first source-side output transistor (upper power transistor) PQ1 and a third sink-side output transistor (lower power transistor) PQ3 provided between the supply Vcc and the ground GND. Similarly, the other output amplifier section has a second source-side output transistor PQ2 and a fourth sink-side output transistor PQ4 provided between Vcc and GND. The load L is then connected in a STL configuration between the +OUT and −OUT output terminals respectively connected to the two output amplifier sections.
A first over-current detector 10 is provided between the base of the source-side output transistor PQ1 and a current detector 500 for supplying detection current to the current detector 500 in accordance with the current flowing through the output transistor PQ1. Similarly, a third over-current detector 30 is provided between the base of the sink-side output transistor PQ3 and the current detector 500 for supplying detection current to the current detector 500 in accordance with the current flowing through the output transistor PQ3. A second over-current detector 20, having the same configuration as the first over-current detector 10 shown in FIG. 1, supplies detection current to the current detector 500 (not shown in FIG. 1) in accordance with the current flowing through the output transistor PQ2, and a fourth over-current detector 40, having the same configuration as the third over-current detector 30 shown in FIG. 1, supplies detection current to the current detector 500 in accordance with the current flowing through the output transistor PQ4.
For example, at the output transistor PQ1, if a short circuit occurs between the +OUT output terminal and GND, an over-current (short-circuit current referred to hereinafter as short-circuit current Iout2) greater than or equal to a predetermined current value Iout2 flows via the output transistor PQ1 from the supply Vcc from the shorted +OUT output terminal to GND as shown by the dashed line in FIG. 1 during operation of the power amplifier. The first over-current detector 10 outputs a detection current in accordance with the short-circuit current Iout2 to the current detector 500.
More specifically, first, the source-side output transistor PQ1 turns on and the short-circuit current Iout2 flows so that the base voltage of the PNP-type source-side output transistor PQ1 drops below Vcc by the amount of an emitter-base voltage VBE. Divider resistors R1 and R2 are connected between the base of the source-side output transistor PQ1 and the supply Vcc. In accordance with the drop in base voltage of the source-side output transistor PQ1, the voltage at node n1 between the divider resistors R1 and R2 drops.
The base of a PNP-type transistor Q1 is connected to the node n1. When the voltage of the node n1 drops and the base-emitter voltage of the transistor Q1 becomes greater than an operating threshold of the transistor Q1, the transistor Q1 turns on. When the transistor Q1 turns on, a current flows from the supply Vcc to the base and emitter of a transistor Q2, via a resistor R5 and the emitter and collector of the transistor Q1, and the transistor Q2 turns on.
The NPN transistor Q2 and an NPN transistor Q6 form a current-mirror circuit. A current Iq6, which is equivalent to a current Iq2 flowing to the transistor Q2, flows to the transistor Q6. The base and collector of a transistor Q5 and the base of a transistor Q9 are connected to the collector of the transistor Q6. The transistors Q5 and Q9 are PNP transistors, their emitters are both connected to the supply Vcc, and the transistors Q5 and Q9 form a current-mirror circuit.
When the transistor Q5 operates and current Iq5 corresponding to the current flowing to the transistor Q6 flows to the transistor Q5, a current Iq9, which is equivalent to the current Iq5, flows to the transistor Q9 and is supplied to the current detector 500 as detection current in accordance with the short-circuit current Iout2
On the other hand, for example, at the output transistor PQ1, if a short circuit occurs between the +OUT output terminal and Vcc, an over-current (short-circuit current referred to hereinafter as short-circuit current Iout1) greater than or equal to a predetermined current value Iout1 flows from the supply Vcc via the output transistor PQ3 to GND as shown by the dotted line in FIG. 1 during operation of the power amplifier. The third over-current detector 30 outputs a detection current in accordance with the short-circuit current Iout1 to the current detector 500.
More specifically, when the sink-side output transistor PQ3 turn on and the short-circuit current Iout1 flows, the base voltage of the NPN sink-side output transistor PQ3 rises according to the short-circuit current Iout1. Divider resistors R3 and R4 are connected between the base of the power transistor PQ3 and GND (ground potential). The base of a NPN transistor Q4 is connected to a node n2 between the divider resistors R3 and R4. Thus, when the power transistor PQ3 turns on, the voltage at node n2 rises, and the base-emitter voltage of the NPN transistor Q4 becomes greater than an operating threshold of the transistor Q4, the transistor Q4 turns on.
The emitter of the transistor Q4 is connected to GND via a resistor R6 and the collector is connected to the base and collector of the transistor Q3. Furthermore, the above-mentioned PNP-type transistor Q3 and a PNP-type transistor Q7, the emitters of which are connected to a common line from the +OUT output terminal, form a current-mirror circuit. Therefore, when the transistor Q4 turns on and accordingly the transistor Q3 turns on, and a current Iq3, which corresponds to the current caused to flow by the transistor Q4 toward GND, flows to the transistor Q3, a current Iq7, which is equivalent to the current Iq3, flows to the transistor Q7. The current Iq7 is supplied to the current detector 500 as a detection current in accordance with the short-circuit current Iout1 via a PNP-type transistor Q8, which is connected as a diode for reverse current prevention.
Similar to the first over-current detector 10 as described above, the second over-current detector 20 is provided between the base of the PNP-type source-side output transistor PQ2 and the current detector 500. If the short-circuit current Iout1 flows to the source-side output transistor PQ2, the second over-current detector 20 operates in the same manner as the first over-current detector 10 and a detection current in accordance with the short-circuit current Iout1 is supplied to the current detector 500.
Furthermore, similar to the third over-current detector 30 as described above, the fourth over-current detector 40 is provided between the base of the NPN sink-side output transistor PQ4 and the current detector 500. If the short-circuit current Iout2 flows to the sink-side output transistor PQ4, the fourth over-current detector 40 operates in the same manner as the third over-current detector 30 and a detection current in accordance with the short-circuit current Iout2 is supplied to the current detector 500.
If even one of the detection currents supplied to the short-circuit current detectors 10, 20, 30, and 40 is greater than or equal to a predetermined value, the current detector 500 generates an over-current detection signal I1 and supplies the signal to a controller 300. When the over-current detection signal I1 is supplied, the controller 300 operates an amplifier control transistor Q10, stops the operation of an output amplifier bias section 400 to stop the signal output from the output transistors PQ1-PQ4, and prevents the over-current from flowing to the output transistors.
In the configuration for over-current prevention in FIG. 1, in comparison to the voltage at the +OUT and −OUT output terminals of the amplifier is approximately Vcc/2 during normal operation, a voltage higher than Vcc/2 is generated when a short circuit occurs between the supply Vcc or GND and either the +OUT or −OUT output terminal as described above. Heretofore, to reliably prevent the generation of over-current, a voltage detector 200 is provided in addition to the above-mentioned current detector 500 in the power amplifier to detect the terminal voltages at the ROUT and −OUT output terminals.
The voltage detector 200 outputs an over-voltage detection signal T2 to the controller 300 when the terminal voltage is greater than or equal to a predetermined value. When the over-current detection signal I1 is output from the current detector 500 and the over-voltage detection signal I2 is supplied from the above-mentioned voltage detector 200, the controller 300 operates the amplifier control transistor Q10 and continues control of the amplifier (latches up the amplifier stop instruction), such as stopping its operation.
Due to the above-mentioned over-current prevention configuration, if a short circuit occurs between the supply terminals or between the supply and the amplifier output, the amplifier output voltage is higher than a normal value of Vcc/2 so that the power amplifier can be reliably protected from over-currents provided the operation of the power amplifier is kept stopped.
However, if a short circuit occurs between the +OUT and −OUT output terminals, over-currents Iout1 and Iout2 flow to the output transistors PQ1-PQ4 and the terminal maintains a voltage of approximately Vcc/2, which is similar to that during normal operation. For this reason, the over-current detection signal I1 is output from the current detector 500 although the over-voltage detection signal I2 is not output from the voltage detector 200. Therefore, when the over-current detection signal I1 is output and the power amplifier initially stops, the generation of over-current is then not detected and the generation of over-current detection signal I1 is terminated. For this reason, the operation of the power amplifier resumes and over-current again flows to the power amplifier.
When over-current flows intermittently to the power amplifier in this manner, this leads to deterioration of the output transistors PQ1-PQ4. Therefore, in such an instance, a set value (threshold) for determining the generation of over-current, for example, a set current value (current threshold) at the current detector 500, can be lowered. However, since the sound may become intermittent if the current detector 500 operates during normal operation and the power amplifier stops, a limitation is that the set current value cannot be set lower than a current value corresponding to a maximum current value Icmax that flows to a load L during normal operation. Therefore, lowering the set current when intermittent over-current is generated to prevent the deterioration of the output transistors PQ1-PQ4 is inadequate.